Fix PatchTree incorrect changeList entries on decode failure

[kennethloeffler]

While I was testing #803, I discovered that PatchTree.build can create malformed changeList entries when a property decode fails. This causes errors when other parts of the system (such as DomLabel) attempt to interact with them. The core of the problem is that we're using next to obtain prop type-value pairs from the encoded property values in the incoming patch. next returns multiple values (in this case, the type followed by the value), which are then passed as-is to addProp or the table cons. What we want instead is to only pass the prop value.

This PR simply uses a select to only pass the prop value. I can also assign the result of next to variables in these cases if we think select is too ugly...

[Dekkonot]

This seems like a fine patch to me, no need to store it in a variable.